when ob/ob mice were fed HFD for 6 weeks, their
body temperature drop triggered by 24 hours
of fasting was reduced to 2.3° ± 1.3°C (n = 6, P <
0.01) (Fig. 2B). Thus, although both the HFD
and ob/ob mice are obese and are widely used
for studies of insulin resistance, the fasting response in HFD-fed wild-type animals differs
from the fasting response in ob/ob mice. Together, these data lend credence to a model in
which leptin signaling participates in fasting-induced temperature declines in a manner blunted
by HFD exposure.

To test whether uridine participates in thermoregulation during fasting and refeeding, we
analyzed plasma uridine levels in HFD-fed ob/ob
and wild-type mice. We detected higher levels
of plasma uridine in ob/ob mice than in wild-type mice; conversely, obese HFD-fed wild-type
mice harbored plasma uridine levels comparable to those in lean chow-fed wild-type mice
(Fig. 2C). Fasting further increased plasma uridine levels in ob/ob mice in the first 4 hours after
food removal, but fasting had no effect on plasma uridine levels in HFD-fed wild-type mice
(Fig. 2C). Upon refeeding, ob/ob mice exhibited
a rapid decline in plasma uridine levels, whereas
no significant changes were observed in obese
HFD-fed wild-type mice (Fig. 2C). These data
reveal a strong correlation between plasma uridine levels and thermoregulation and further
highlight the distinct effects of HFD and leptin-deficiency on body temperature that is mediated through plasma uridine.

Uridine-triggered temperature declines in rodents rely on the activity of uridine phosphorylase (8), the enzyme responsible for initiation
of uridine catabolism. We hypothesized that
increases in plasma uridine levels during fasting
mediate the temperature drop by increasing uridine availability for degradation. To test this, we
injected ob/ob mice with N-(phosphonacetyl)-L-
aspartate (PALA). This compound is an inhibitor of aspartate transcarbamylase (14), which is
the rate-limiting enzyme for uridine biosynthesis
(15) and is part of the trifunctional protein Cad
(carbamoyl phosphate synthetase 2, aspartate
transcarbamylase, and dihydroorotase). In this
context, PALA prevented both the drop in body
temperature in ob/ob mice (Fig. 2D) and the elevation of plasma uridine after 24-hour fasting.
These findings support a model in which plasma
uridine levels govern core body temperature.

Uridine links thermoregulation
with leptin

Temperature exchange with the environmentdepends on the difference in temperature be-tween the subject and surrounding environmentand relies on three mechanisms: conduction,convection, and radiation. To examine whetherthe acute temperature drop triggered by uri-dine is mediated by temperature exchange withthe environment, we housed mice in a near-thermoneutral environment (29°C), 7°C abovethe ambient room temperature used for thestudies depicted in Fig. 2A. Wild-type mice in-jected with PBS showed a slight increase inbody temperature after they were moved to 29°C(37.5° ± 0.24°C, 38.3° ± 0.2°C, and 38.1° ± 0.2°C at 0, 15, and 30 min, respectively; n = 6)(Fig. 3A). When mice were injected with uri-dine and subsequently transferred to the 29°Cincubator, the temperature drop seen at ambientroom temperature (Fig. 2A) was no longer ob-served; rather, the mice displayed a body temper-ature only slightly lower than that observed withthe PBS injection under the same conditionsDeng et al., Science 355, eaaf5375 (2017) 17 March 2017 2 of 9